LNG or Liquefied Natural Gas results from a process in which natural gas is cooled until it becomes a liquid. Liquefying natural gas allows for economical transport of natural gas when pipeline transport is cost prohibitive.
Refrigeration processes for LNG generally require refrigeration compressors and compressor drivers and, depending on the size of the LNG facility, there may be multiple refrigeration compressors and drivers.
Currently, LNG production is dependent on industrial gas turbines for mechanical power to drive the refrigeration compressors used to cool or refrigerate the natural gas. Most industrial gas turbines that are used in LNG facilities were originally developed for the electrical power industry and adapted for use as compressor drivers. These gas turbines are either designed for the 50 Hz electrical market in which the gas turbine operates at 3000 rpm or for the 60 Hz electrical market with operating speed of 3600 rpm. Speed variation in power generation is not allowed because of the precise requirement of 50 Hz or 60 Hz power. Any deviation from 3000 rpm or 3600 rpm for 50 Hz and 60 Hz power, respectively, will result in significant problems for electrical users. For example, if the turbine and associated compressor are operating at 3100 rpm, but the electrical grid frequency is 50 Hz (3000 rpm), the generated frequency would be 51.7 Hz. Generating electrical power at 51.7 Hz causes significant problems for users connected to the electrical system. Typically, electrical systems have a frequency tolerance of only ±0.5 Hz. Nevertheless, typical gas turbines designed for power generation do have some frequency variability, on the order of ±5%. This is useful in maximizing LNG production. Another characteristic of such gas turbines is that they are designed to be most efficient when operating at their maximum power. Fully loading the turbine at any speed allows operation at peak efficiency and reduces emissions and specific fuel consumption. The fuel flow rate can be varied to increase or decrease the amount of mechanical energy the turbine produces. Increasing and decreasing the fuel flow rate can be done such that the gas turbine shaft speed remains constant, e.g., 3,000 or 3,600 rpm. Operating a gas turbine at lower fuel rates will significantly reduce its efficiency and increase its emissions.
Starting a gas turbine that is used for power generation is relatively easy because the generator is not energized and the only power required is the power to spin the gas turbine and generator up to operating speed. Once at operating speed, the starter is disengaged and the gas turbine takes over and powers the generator. On the other hand, starting a gas turbine that is used as a compressor driver is much more demanding compared to starting a gas turbine driving an electrical generator. In general, the power required to start a gas turbine and compressor is much greater because of the compression load. The refrigerant is flowing through the compressor during the starting process and the power required increases substantially with increasing speed. A large starting motor is required to spin the turbine and compressor up to operating speed. Typically, this starter motor is an electric motor.
In a typical LNG refrigeration configuration, illustrated in FIG. 1, a common drive shaft 5 connects the gas turbine 2 to one end of the compressor 3 and the starter motor 1 to the other end of the compressor 3. The three connected devices are referred to as a compression string and multiple compression strings are referred to as an LNG train.
To avoid the drive train shock of an “across the line” startup, a frequency converter 4 is used between the electrical power supply and the starter motor 1. The starter motor 1 is gradually brought up from 0 Hz to the line frequency (50 or 60 Hz, as the case may be). A popular type of frequency converter for such applications is called a Variable Frequency Drive, or “VFD.”
Once the starter motor has accelerated the string to the desired operating speed, the gas turbine takes over and provides all the necessary shaft power. At that juncture, the electricity to the starter motor is turned off, and the motor is allowed to “free-wheel.” In some LNG plants, the starter motor is also used, as needed, to provide additional shaft power while the gas turbine and compressor are at operating speed. Adding shaft power while the gas turbine is operating is referred to as “helper” duty.
The primary reason for the helper function is that gas turbine output power depends on the ambient conditions. As the ambient temperature increases, the air density decreases and therefore the gas turbine power decreases. Conversely, as the ambient temperature decreases, the gas turbine power increases. Therefore, LNG production will tend to decline in the warmer months, and increase in colder months. Smaller production variations will occur over the course of a 24-hour period as the temperature rises during the day and falls at night. The helper function may be used to maintain constant LNG production rates by providing additional power. The helper function is needed only in the warmer part of the year and daytime when gas turbine power is reduced. During the cooler part of the year and at night, the gas turbine may be producing excess mechanical power. During such times, the practice has been to reduce the fuel flow rate to the gas turbine enough to eliminate excess power production (maintaining rotational speed) and accepting non-optimum gas turbine operating efficiency. However, Kikkawa discloses (in U.S. Pat. No. 5,689,141) a method for converting the excess mechanical power to electrical power by using the starter/helper motor as an electrical generator. No major alterations are required to make an electric motor reversible so that it can also function as an AC generator. The converted excess turbine power can then be transferred to the electric power supply grid, which may be external or—in the case of many LNG plants—self-generated using the available natural gas as fuel. The generated electrical power reduces the LNG plant's electrical power needs.
The turbine can be sized to provide the power the associated compressor requires during the warmest part of the year. When the temperature drops and less power is required, the turbine can continue to operate at its maximum power output where it is most efficient, with excess mechanical energy converted to electrical power by the starter motor operating in generator mode. (The fuel/air mixture for the gas turbine is readjusted as the temperature changes.)
Kikkawa advocates operating the compression string at the precise rotational speed of 3,000 rpm (if the grid frequency is 50 Hz) or 3,600 rpm (if the grid frequency is 60 Hz) so that the frequency of the electricity that is generated matches the electrical system frequency. This may be called “synchronous” operation. Kikkawa recognizes that non-synchronous operation is an alternative, with a frequency converter used to change the frequency of the generated power to the grid frequency. That frequency converter would be the same one used to provide gradual startup when the starter motor is used to bring the LNG train up to operating speed. However, Kikkawa concludes that such increased use of the frequency converter, an expensive device, would necessitate having a spare frequency converter. Kikkawa teaches synchronous operation to avoid this significant, added capital investment. Kikkawa's method allows the gas turbine to be operated at its most efficient, but provides no throughput control, i.e., the gas turbines in his arrangement can operate at maximum power at synchronous speed but not maximum power at any other speed. Furthermore, Kikkawa makes no allowance for maintaining stability during transient periods of compressor string operation. The electrical connection between the propane and mixed refrigerant compression strings has its mechanical equivalent in a rigid coupling, making stable operation more difficult to control. The configuration does not include the capability to send excess power to the grid for use in other parts of the plant or outside the plant during these transient periods.